The Selective Trimerisation of alpha-Olefins with Chromium Triazacyclohexane Catalysts

Abstract

Described herein is research into the production of hydrocarbons suitable for various industrial applications via α-olefin trimerisation. This work has been split into three principal chapters that each describe a different aspect of the research carried out. In addition, a comprehensive review is presented into the development of selective oligomerisation, with an emphasis on the role of metallacyclic intermediates.α-Olefin Trimerisation: Catalyst Synthesis and Optimisation The synthesis and characterisation of eight novel chromium triazacyclohexane catalysts is described, including a series of catalysts containing alternative abstractable anions. All were shown to be active towards trimerisation, with optimisation resulting in exceptional selectivities of over 95 w%, with the remainder consisting of isomerised LAO and dimer. Investigation of the ‘halogen’ effect indicated the inducement of catalyst decomposition via oxidation. Mechanistic Insights into α-Olefin Trimerisation The proposed metallacyclic mechanism was investigated using a variety of techniques. The regioisomers of the trimer were identified down to an abundance of 0.1% with the use of 13C labelling. All matched those expected, allowing the relative abundance of differently substituted intermediates to be determined. 2H labelling identified competition between elimination pathways, which can only occur for metallacyclic intermediates. Characterisation of the catalyst activation products supported the mechanism, while chain transfer reactions were also identified. Air sensitive mass spectrometry led to observation of peaks matching those expected for proposed intermediates.Ethylene Trimerisation: Process Optimisation and Product Identification Optimisation of experimental procedures and increasing the catalyst bulk led to selectivities of 93 w% and activities beyond 250,000 mol(C2H4) mol(Cr)-1 h-1. Kink kinetics resulted in three distinct periods of differing ethylene consumption for which a mechanism has been proposed. Tetramerisation was detected for these systems for the first time alongside several co-trimerisation products. The C10 species were most abundant and have been fully characterised, supporting the proposed co-trimerisation mechanism.